National Institute of Environmental Health Sciences

By David Richards

Ticks are hardy parasitic arachnids found throughout the world. Changes to global temperatures and precipitation, exacerbated by climate change, may affect tick populations and their habitats, according to a recent article. The article, published October 2020 in the Journal of Medical Entomology, considers the effects of climate change on ticks and tick-borne diseases (TTBD).

“Ticks are inherently climate sensitive,” said lead author Nicholas Ogden, BVSc, D.Phil., a senior research scientist at the Public Health Agency of Canada. “We expect climate change to change the entomological risk from tick-borne diseases globally, and the way it does that is by changing tick survival, the duration of their life cycle, and other factors.”

Ixodes ticks, characterized by their hard bodies, are important vectors for diseases. In North America, they are commonly known as blacklegged ticks or deer ticks that can carry the bacterium Borrelia burgdorferi, which causes Lyme disease. In the U.S., Lyme disease affects approximately 300,000 Americans each year. They can also transmit other lesser known TTBDs, such as babesiosis, an infection of red blood cells similar to malaria, or anaplasmosis, a bacterial disease, and Powassan encephalitis, a viral infection of the brain.

While these ticks were generally confined to mild and humid climates of the mid-Atlantic, southern New England, the Great Lakes of the eastern U.S., and the Pacific Coast of the western U.S., they, along with their diseases, are now being found at higher altitudes and latitudes. This upward climb, not only in North America but also around the world, could be attributed to climate change, according to the article.

Weather and Climate Sensitivity of Ticks

Global temperature and precipitation changes are directly impacting ticks’ survival in new and existing regions and indirectly influencing their development and behaviors. As temperatures and precipitation increase in northern regions of the northern hemisphere, the southern regions are likely getting warmer and, in many locations, drier. Humidity is particularly critical for ticks, as they hydrate themselves by secreting saliva that absorbs water from the air. Warmer than usual temperatures in northern regions may benefit tick populations, while extreme heat in southern regions may cause populations to die from heat exhaustion.

“As it warms, more and more of Canada has become suitable for the tick,” said Ogden. “This is by virtue of it being warm enough for the life cycle to get shorter for the tick population to survive. This will result in more ticks in places where temperatures were previously suboptimal but also change their geographic range poleward and altitudinally.”

Many hard-bodied ticks can endure colder temperatures due to living in insulated woodland habitats and their ability to produce an antifreeze protein; however, cold temperatures lengthen the period of time to reach their next life cycle (the four stages of a tick’s life are egg, larva, nymph, and adult). The article suggests longer warm seasons shorten this period, increase the chances of larvae and nymphs to find hosts, and ultimately increase the likelihood they advance to their next stage.

Pathogen Transmission

To reach new geographic areas, a tick’s journey relies on a process called questing, where the tick holds on to a leaf or blade of grass with its back legs and outstretches its front legs in anticipation for a host – like mice, lizards, birds, deer, dogs, or humans – to walk by for it to latch on. Ogden explained, when attached, the tick injects a cocktail of substances to modulate the host’s immune and inflammatory response, allowing it to feed undetected.

Bacterial and viral pathogens, like ticks, rely on living hosts. As with many TTBDs, larvae do not inherit the pathogen at birth but rather become infected by their first host, such as rodents, that are simultaneously becoming infected by nymphs. This receiving and transmitting stage is critical for the pathogen’s survival.

“In general, when ticks spread their geographic range, they bring pathogens with them,” said Ogden. “Now you have new populations of animals and humans who are exposed.”

For Ixodes ticks in North America, the periods when both larvae and nymphs are active differ geographically and do not always line up, though the article suggests regional temperature changes can affect that. However, the reality is more complex. Unrelated to climate change, changes to day length also have shown to influence diapause, a physiological behavior in ticks that affects development and activity, according to the article.

Knowledge Gaps

Understanding ticks and their disease transmission in relation to climate change is critical to protecting and planning for public health, especially in regions where pathogens are not endemic. For example, in Canada, especially in the provinces of Ontario, Québec, and Nova Scotia, counts of Lyme diseases cases increased from 144 in 2009 to 992 in 2016.

Ogden emphasized the need for a more collaborative and coordinated approach to the issue. Combined efforts in field observations, laboratory analysis, and public health modeling can produce preventative and predictive tools such as risk maps to track geographic spread of ticks and tick-borne diseases.

NIEHS, as part of its leadership within the U.S. Global Change Research Program (GCRP), has supported scientific activities related to promoting understanding of climate change impacts on vector-borne and other infectious diseases and the development of preventative and predictive tools. For example, NIEHS’ John Balbus, M.D., Senior Advisor for Public Health, was part of the steering committee for the GCRP report “Predicting Climate-sensitive Infectious Diseases to Protect Public Health and Strengthen National Security”. Balbus has also presented on GCRP efforts to develop vector-borne disease seasonal forecast models at the American Geophysical Union Annual Meeting. More recently, NIEHS hosted a webinar on geospatial datasets to support research on climate, environment, and infectious disease connections. The webinar was produced in partnership with the Models of Infectious Disease Agent Study (MIDAS), which is funded by the National Institute of General Medical Sciences and the National Oceanic and Atmospheric Administration.

“If you have no knowledge of how climate change might be impacting levels of risk, then you are not going to be able to plan for tick-borne diseases,” said Ogden. “People and animals are going to get infected without any controls in place. It is critical to understand how climate change and other factors are determining where and when ticks are present in order to form a public health and animal health response.”